CHAPTER 3

Introduction

Life processes are being carried out by living cells at all times to enable them to stay alive. Examples of these processes are absorbing water and nutrients (minerals, ions, glucose and amino acids from the surroundings), excreting waste products (urea and uric acid that are not needed by the cells) and exchanging respiratory gases (oxygen and carbon dioxide) during respiration

Exchanging respiratory gases during respiration happen in alveolus

The cells absorb certain substances to enable them to carry out biochemical reactions. Examples of the biochemical reactions they undergo are respiration and photosynthesis. For example, animal and plant cells need oxygen for respiration while plant cells also need water, carbon dioxide and light to carry out photosynthesis.

Cells also need to excrete waste products produced within their internal environment. Examples of substances that have to be excreted by plant cells are excess oxygen and waste products such as carbon dioxide. Waste products that are formed during biochemical reactions within the cells must be excreted because they are poisonous and can cause harm to the cells.

Necessity for Movement of Substances Across the Cell Membrane

The movement of substances in and out of cells is necessary as this helps the cells to obtain substances they require such as the oxygen (animal cell), carbon dioxide (plant cell), glucose and other nutrients.

The movement also allows the excretion of wastes; this enables the cells to maintain a suitable pH level and ionic concentration for them to perform their enzyme activities. The movement of substances also allows the exchange of gases and the transfer of substances between cells.

The movement of substances in and out of cells is conducted through the cell membrane known as the plasma membrane. It is basically a boundary that separates the internal environment of the cells from the external environment that surrounds them

Figure below shows the movement of substances in and out of the cells through the plasma membrane. The plasma membrane regulates the exchange of substances between the content of a cell and its external environment.

The Structure of The Plasma Membrane

How does the structure of the plasma membrane
that allows substances to move in and out of a cell, look like? The structure of this membrane can be explained by a membrane model called the fluid mosaic model
proposed by Singer and Nicholson in 1972.

In this model, the plasma membrane consists of a phospholipid bilayer
, in which carrier and pore proteins are embedded. This bilayer surrounds the internal environment of the cell and thus separates the cytoplasmic fluid of the cell from the external environment.

Movement of Substances Across the Plasma Membrane

The movement of substances across the plasma membrane can be divided in two parts – passive transport and active transport.

Passive transport

Passive transport is the movement of substances across the plasma membrane from a region of high concentration
to a region of lower concentration
. In passive transport, no energy is required
by a cell to move the substances through the cell membrane. The following are examples of passive transport:

i.	simple diffusion
ii.	facilitated diffusion
iii.	osmosis

Active Transport

Active transport involves the movement of ions and molecules against the concentration gradient. So this transport is like the pumping process of molecules or ions across the plasma membrane from a region of low concentration to a region of high concentration. The mechanism is similar to facilitated diffusion where carrier proteins are used except that the movement of molecules is now against the concentration gradient.

Since it requires pumping and taking the ions or molecules across the cell membrane against the concentration gradient, energy is required by the cell. The cell must use its own internal or metabolic energy to transport the ions or molecules across its membrane. The energy is provided by ATP.

The mechanism of active transport for sodium ions from the inside of the cell (lower concentration) to the outside of the cell (high concentation) across the plasma membrane 

Process of Passive Transport and Active Transport

Simple Diffusion

An example of simple diffusion is the gaseous exchange in the alveoli and blood capillaries during respiration

i.	Oxygen in the alveoli diffuses across the alveolar and capillary walls into the blood capillaries of the lungs. This is because the concentration of oxygen in the alveoli is higher than that in the blood capillaries.
ii.	Carbon dioxide diffuses across the capillary and alveolar walls from the blood capillaries into the alveoli. This is because the concentration of carbon dioxide in the blood capillaries is higher than that in the alveoli.

Other processes of simple diffusion are the gaseous exchange through the stomata of leaves during photosynthesis, the gaseous exchange in unicellular organisms and the evaporation of water through the stomata from cell leaves during transpiration.

The Effect of Hypotonic, Hypertonic, and Isotonic Solutions on Plant Cells 

A Plant Cell in A Hypotonic Solution

The external solution has a lesser concentration of the solute than that in the cell sap. So, the external solution has a greater concentration of water molecules compared to the concentration of water molecules in the cell sap.

The water molecules move into the cell by osmosis and push the cell contents outwards, against the cellulose cell wall. The pressure outward onto the cell wall is called the turgor pressure. This pressure makes the cell very turgid, rigid and firm. This turgidity provides the plant with its mechanical support, enabling it to stay upright.

When the cell is turgid, water molecules will diffuse into and out of the cell at the same rate (dynamic equilibrium).

So, the effect of the hypotonic solution is to increase the size and volume of the cell, making it rigid and turgid.

A Plant Cell in A Hypertonic Solution

The external solution has a greater concentration than that of the solute in the cell sap. So, the external solution has a lesser concentration of water molecules than the concentration of the water molecules in the cell sap.

The water molecules diffuse out of the central vacuole and cytoplasm of the cell via osmosis. The vacuole shrinks and becomes smaller. The cytoplasm also shrinks, becoming smaller in size and is pulled away from the cell wall. The cell loses water and becomes flaccid, causing the plant to soften and wilt.

The process whereby the cell loses water is called plasmolysis. So, plasmolysis is the loss of water from the cell by osmosis. This becomes evident when the cell contents pull away from the rigid cell wall as the water moves out and the plant becomes soft and less turgid. If the plasmolysis continues, the cell will die and in turn, the plant itself will also die

A Plant Cell in An Isotonic Solution

Water, solute and other substances are stored mainly in the large central vacuole. In the plant cell, the plasma membrane is enveloped by a wall made of cellulose. The external solution has the same concentration of water molecules as that of the water molecules in the cell sap. So, water diffuses into and out of the cell at the same rates.

The original size of the cell is not changed because the net movement of water across the plasma membrane is zero. Its volume and size remain the same. Hence, the isotonic solution does not affect the cell size and shape.

An Animal Cell In A Hypotonic Solution

The external solution has a lesser concentration of solute than that in the cell sap. So, the external solution has a greater concentration of water molecules compared to the concentration of water molecules in the cell sap.

The water molecules move into the cell by osmosis, inflating and swelling the cell and finally, rupturing it. This is because the animal cell has a very thin wall and this wall is unable to withstand the strong osmotic pressure developing within the cell. The bursting of the cell by the diffusion of water into the cell is called haemolysis.

An Animal Cell In A Hypertonic Solution

When a red blood cell is placed in a hypertonic solution, the external solution has a greater concentration of solute that that in the cell sap. So, the external solution has a lesser concentration of water molecules than the concentration of water molecules in the cell sap.

Water diffuses outside the cell by osmosis. The cell loses water to the external environment, and this causes the cell to shrivel and the plasma membrane to crinkle. This situation is called a crenation. If the cell continues to lose its water contents, the cell will probably die.

An Animal Cell In An Isotonic Solution

When an animal cell is immersed in an isotonic solution, the original size of the cell is not changed because the net movement of water across the plasma membrane is zero. This is because the external solution has the same concentration of water molecules as that in the cell sap. So, water diffuses into and out of the cell at the same rates. The cell retains its size and the shape remains in the form of a biconcave disc

The Effect and Applications of Osmosis in Everyday Life

We shall now look at the applications of osmosis in everyday life. Two aspects of the application will be considered, namely, the wilting of plants and the preservation of food.

Wilting in Plants

The excessive use of chemical fertilisers usually causes the wilting in plants. The soil water becomes more concentrated and hypertonic compared to the cell sap of plant roots. This is because fertilisers, such as potassium nitrate which are added to the soil, dissolve in soil water. In this situation, water diffuses from the cell sap into the soil by osmosis and results in the cell being plasmolysed. A plant becomes wilted because the flaccid cells cannot provide support anymore.

In addition, the wilting in plants may also be caused by a shortage of water. When the soil dries up, the soil water becomes more concentrated and hypertonic. So, plants lose water by osmosis and the cells become flaccid.

Wilting refers to the loss of rigidity in non-woody and herbaceous plants. This situation occurs as a result of the diminishing water content in the cells and the turgid pressure in the non-lignified plant cells falls towards zero. Within a short period, the cytoplasm of a plant cell is damaged by plasmolysis. A wilted plant will eventually die if the period of plasmolysis is prolonged.

When water is made available, the cells will promptly recover and the plant gains its turgidity.

Food Preservation

Food preservation is the process of treating and handling food; the concept of osmosis and diffusion is applied during this process. Food preservation is the process conducted to stop or greatly slow down the spoilage of food, preventing food-borne diseases while maintaining the nutritional value, density, texture and flavour of the food.

Natural preservatives such as salt, sugar and vinegar can be used to preserve different types of food such as fruits, vegetables and fish. When these preservatives are added to the food, the surrounding solution becomes hypertonic compared to the contents of the food. Water leaves the food by osmosis and preservatives enter the cell sap.

Preservation prevents the growth of bacteria, fungi and other micro-organisms which can spoil the food. In this way, the food will have a longer shelf life.